ALEX Salmond has demanded Danny Alexander apologise for a missing “mountain of black gold” as the two traded blows over oil figures.

The Lib Dem Treasury minister had accused Salmond of offering a “fantastical picture” of what ­an ­independent Scotland’s oil-pumped finances would be.

Alexander hit out after the UK Office for Budget Responsibility ­drastically revised down the amount of cash it expects to be raised from North Sea oil and gas revenues.

The independent body now ­forecast revenues of £61.6billion over the next three decades – down from £82.2billion.

But the First Minister dismissed the OBR’s figures as “stuff and nonsense”.

A spokesman for the First Minister said: “Danny Alexander must ­apologise for the Treasury’s dodgy dossier on the finances of an ­independent Scotland, just as he should apologise for his leading role in Westminster’s 2011 tax grab on the North Sea, which he boasted about before being forced to scrap it.

“North Sea oil is a huge asset and will be for many decades to come – and the OBR’s forecasts rest on ­estimates of future production which are well below those used by the industry, by leading experts and by the UK Government.

“Instead of acting as a frontman for a Tory chancellor and an ­apologist for Tory cuts, Danny Alexander should come clean with the people of Scotland on Westminster’s plans to slash Scotland’s cash in the event of a No vote.”

But Alexander said the SNP leader was “promising milk and honey” when he should be spelling out the “painful choices a separate Scotland would have to make” under ­independence.

Chief Secretary to the Treasury Danny Alexander (Image: PA)

Alexander, the Chief Secretary to the Treasury, said with voting only weeks away, Salmond is “still making plans based on untold oil wealth beyond anything independent forecasters consider plausible”.

In a letter to the SNP leader, ­Alexander wrote: “This stage in the referendum campaign should be about presenting the people of ­Scotland with the facts about what separation would mean. Instead you persist in offering a fantastical picture of a separate Scotland’s public finances.”

He said: “It is clear to every ­independent expert that there is no boom in oil revenues on the way, and because extraction is expected to become more expensive, we have to be realistic about tax revenue ­projections.

He added: “This is unfair to the electorate – it is incredible that so close to the referendum you are still promising milk and honey when you should be laying out the real facts about the painful choices a separate Scotland would have to make.”